1. Field of the Invention
The present invention relates to the field of electronic amplifier circuits.
2. Description of Related Art
The growth of technologies such as wireless communication has led to an increasing need for high performance electronic amplifiers that have high gain and low noise.
FIG. 1 shows a typical example of a low noise RF amplifier known as a cascode amplifier. The cascode amplifier includes a first MOSFET transistor 10 and a second MOSFET transistor 12 that are connected in series between a load impedance Zload driven by a voltage source Vdd and a common potential (referred to herein as ground). The first transistor 10 receives an input signal RFin at its gate along with a DC bias voltage supplied by a bias circuit 14. The input signal RFin is amplified in the first transistor 10 to produced an amplified signal at the drain of the first transistor. The amplified signal is received at the source of the second transistor 12 and is conducted through the second transistor 12 to an output node where an output signal RFout is presented. The second transistor 12 typically does not amplify the signal provided by the first transistor 10, but rather is used to prevent the output node from seeing parasitic capacitances of the first transistor, thus lowering the output impedance of the amplifier circuit and improving its frequency response.
As seen in FIG. 1, the substrates of the MOSFET transistors 10, 12 are coupled to their respective sources. This arrangement is utilized to eliminate the substrate bias effect or “body effect,” in which the MOSFET substrate (also referred to herein as the “body”) acts as a second gate that influences carrier availability in the MOSFET channel region. The body effect is explained in more detail with reference to FIGS. 2 and 3.
FIG. 2 shows a cross-section of a typical n-type MOSFET comprised of a p-type substrate 20 in which n-type source and drain regions 22, 23 are formed at opposing sides of a channel region 24. For purposes of the present explanation, the channel region 24 is shown as having an n-type inversion layer formed therein. A gate dielectric 26 lies between the channel region 24 and a gate electrode 28. A depletion layer 29 separates the p-type and n-type regions. The thickness of the depletion layer with respect to the other elements is exaggerated for purposes of illustration.
The drain current of the MOSFET is controlled by modulating the availability of majority carriers (in this case, conduction band electrons) in the channel region between the source and drain. Carrier availability is largely controlled through a capacitive effect that is caused by application of a voltage to the gate 28. Consequently, variations in the gate voltage produce corresponding variations in carrier availability that cause the drain current to be modulated in a manner that corresponds to modulation of the gate voltage. However, carrier availability is also affected in a similar manner by any voltage applied to the body of the MOSFET. Thus, the MOSFET is typically modeled in the manner shown in FIG. 3. In this model, the behavior of the MOSFET is approximated by a pair of parallel connected current sources 30, 32 that produce a drain current Id. The first current source 30 represents the effect of the gate voltage Vg on carrier availability, which produces a current having a magnitude approximately equal to the MOSFET transconductance Gm times the gate-source voltage Vgs. The second current source 32 represents the effect of the body voltage Vb on carrier availability, which yields a current having a magnitude approximately equal to the body effect transconductance Gmb times the source-body voltage Vsb. As shown by this model, the application of a reverse bias to the source-body junction (i.e., a voltage that widens the depletion layer 29) has an effect that is equivalent to the generation of a current in the channel region that is opposite in polarity to the current produced in response to the gate voltage, resulting in an over-all reduction in the drain current produced in response to a given gate voltage. Since the MOSFET body is typically held at a fixed voltage, the body effect is generally understood to reduce the transconductance of the MOSFET or to increase the threshold voltage of the MOSFET. In order to avoid this effect, MOSFET circuits such as the cascode circuit of FIG. 1 connect the source directly to the body so that the source-body voltage is zero.